In a conventional type of injector, a valve member formed as an integral part of a movable core is driven using magnetic attraction generated between a fixed core and the movable core in response to energization of a coil. In such an injector, the valve member moves back and forth in the axial direction according to whether or not the coil is energized. Consequently, when the movable core moves towards the fixed core, it collides with the fixed core, whereas when the movable core moves away from the fixed core, the integral valve member collides with the valve seat. As a result, the impact of the collisions causes so-called bouncing of the movable core and the valve member.
In an injector, bouncing of the valve member results in variation of opening time and closing time of the injection nozzle. This results in uncontrollable and irreproducible injection of fuel from the injection nozzle. The effect of bouncing is particularly marked when the length of the energizing pulse applied to the coil is small, making it impossible to precisely control the amount of fuel injected and the shape of the fuel spray. Accordingly, an injector has been proposed in which two stoppers are provided on the valve member, with the movable core disposed between these stoppers (see Published Japanese Translation of PCT application No. 2002-528672).
In the injector disclosed in the Published Japanese Translation of PCT application No. 2002-528672, the movable core is able to move in the axial direction between the two stoppers. Consequently, when the valve member collides with another member, opposing inertial forces are generated in the valve member and the movable core. This moderates the impact force at the point of collision. In addition, by providing buffer springs between the movable core and the stoppers, the impact of the collisions is moderated, and the occurrence of bouncing is reduced.
However, with the technology disclosed in the Published Japanese Translation of PCT application No. 2002-528672, two stoppers must be provided in the valve member, and the movable core must be interposed between the two stoppers in such a manner as to be movable relative to the valve member. In addition, buffer springs must be provided between the movable core and the stoppers. This leads to a more complicated construction and increases the some number of components. Furthermore, long term operation of the injector can cause spring fatigue and abrasion and the like. Consequently, the characteristics of the springs vary over time, and it is difficult to ensure stable fuel injection characteristics over an extended period.